Thin-film hard disc magnetic media are widely used for on-line data storage in computers. In recent years, considerable effort has been spent in achieving higher recording density. Among the most important factors in this recording density are the following parameters.
(1) Magnetic remanence is a measure of the signal amplitude which can be read from an isolated pulse stored in the medium--the greater the remanence, or moment, the greater the signal amplitude which can be detected in a reading operation. PA1 (2) Coercivity is defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density. PA1 (3) Signal/noise ratio (SNR) is defined as the ratio of the power of a given signal to the noise power in a given bandwidth, and provides a measure of signal amplitude relative to noise level. The higher the S/N ratio, the greater the bit density which can be read with a given degree of reliability. PA1 (4) Overwrite is defined as the effectiveness in erasing a signal read at one frequency with a higher frequency signal. Overwrite provides a measure of the residual signal which remains after overwrite by a new signal. PA1 (5) Flying height is the distance at which a read/write head floats above the spinning disc. Less overlaps of voltage signals in adjacent magnetic domains in the disc occurs as the read/write head is moved closer to the disc surface, allowing recording density to be increased. The flying height is limited principally by surface irregularities in the disc. For a thin-film medium formed on a standard aluminum substrate, the minimum flying height is about 6 micro-inches. Flying heights as low as 1-2 micro-inches can be achieved in a thin-film medium formed on more perfect-surface substrate, such as a glass substrate. PA1 (6) Magnetic hysteresis loop squareness is the ratio of coercivity to saturation field. As the saturation field becomes smaller and approaches the coercivity, it takes less field strength to switch or "write to" the medium. PA1 (7) Bit shift or peak shift is a term that refers to the broadening between voltage peaks, as well as the reduction in peak amplitude, that occurs in the read voltage waveform. It is generally desired to achieve low bit shifting inasmuch as bit shifting limits the resolution at which adjacent peaks can be read, and thus places an upper limit on recording density. PA1 (8) Bit density is the density in the medium at which bits are recorded. A bit is identified as a flux transition in the medium, and bit density is measured as the number of flux transitions per unit length. Typically, the higher the bit density, the lower the signal-to-noise ratio. PA1 (9) Pulse width is a parameter that is generally inversely related to the coercivity. The higher the coercivity, the narrower a bit needs to be sensed. PA1 (10) Signal amplitude, or peak-to-peak amplitude of a single pulse is a function of recording frequency. The recording density of the medium is related to the drop in the signal amplitude at increasing recording frequency. PA1 (11) Signal resolution is the ratio of high-frequency track average amplitude divided by the low-frequency track amplitude. The recording frequency at which 70% resolution is achieved represents one measure of information storage density on a disc.